1. Field of the Invention
The present invention relates to a device for injection molding articles of plastics material having hollow spaces. The device includes a molding tool with a mold cavity. Plastics material melt is supplied to the mold cavity through a nozzle. Another pressurized medium, such as air, gas or liquid, can be supplied to the mold cavity through a hollow needle at a location different from the location of the nozzle.
2. Description of the Related Art
It is already known in the art from German Offenlegungsschrift 21 06 546 to manufacture articles having hollow spaces of plastics material in the mold cavity of a molding tool by supplying, after the injection of the plastics material melt within the injection period provided for this purpose, to the low-viscosity center of the injection molded body within the subsequent afterpressure period compressed gas, preferably compressed air, and then to fill the hollow space in the injection molded body during the afterpressure period with a foamed plastics material. For this purpose, an injection mold with injection nozzles is used which has another nozzle for supplying the afterpressure gas and which can be connected to an appropriate gas source and whose opening projects into the region of the low-viscosity center of the injection molded body.
In this manner, it is possible to produce articles of plastics material, even with large volumes, without problems. Without requiring a complicated construction of the molding tools and without having to operate with exactly measured injection periods, afterpressure periods and exact pressures and temperatures.
On the other hand, it is also already known from German Auslegeschrift 24 61 580, from German Offenlegungsschrift 25 01 314, and from U.S. Pat. Nos. 4,101,617 and 4,740,150 to inject the plasticized plastics material, on the one hand, and the additional pressurized medium, on the other, practically at the same location into the mold cavity of the molding tool.
Particularly in injection molding devices of the type mentioned last, it has been found disadvantageous that the nozzles serving for supplying the additional pressurized medium, such as air, gas or liquid, are too frequently clogged with the plastics material to be processed and, therefore, have to be cleaned after almost every successive injection procedure to ensure that they satisfactorily carry out their functions.
The device according to German Offenlegungsschrift 21 06 546 has a nozzle which is formed as a hollow needle extending significantly into the mold cavity. However, this hollow needle is also clogged in an undesirable manner, at least when the amount of the plastics material melt introduced into the mold cavity is dimensioned such that it reaches within the low-viscosity center of the injection molded body up to the opening of the hollow needle, or even covers or flows over the opening when the gas pressure is reduced in the hollow space as the afterpressure period ends. This is because any melt still in liquid form can penetrate into the relatively large cross-sectional area of the opening of the hollow needle and clog the opening.
If this disadvantage is to be prevented in the device according to German Offenlegungsschrift 21 06 546, the hollow needle must always extend into the center of the mold cavity by such an extent that any liquid plastics material melt cannot reach the nozzle opening even after the afterpressure period has ended, i.e., it must be ensured that no melt can reach the nozzle opening.
However, it has been found advantageous if the nozzles or hollow needles serving for supplying the additional pressurized medium do not extend through the wall thickness of the articles to be manufactured in the cavity of the molding tool up to the region of the hollow space to be formed; rather, the nozzles or hollow needles should only project into a portion of the wall thickness being created. This facilitates an automatic sealing of the opening of the nozzle or hollow needle at the end of each injection procedure from the inside by means of plastics material melt still in liquid form, as is desirable or even required in most cases.